The present invention relates to signal communications and, in particular, to frequency hopping based signal communications.
One type of communications channel for which usage is expanding particularly rapidly is wireless communications, particularly as more radio spectrum becomes available for commercial use and as cellular phones become more commonplace. In addition, analog wireless communications are gradually being supplemented and even replaced by digital communications. In digital voice communications, speech is typically represented by a series of bits which may be modulated and transmitted from a base station of a cellular communications network to a mobile terminal device such as a cellular phone. The phone may demodulate the received waveform to recover the bits, which are then converted back into speech. In addition to a growing demand for voice communications, there is also a growing demand for data services, such as e-mail and Internet access, which typically utilize digital communications.
There are many types of digital communications systems. Traditionally, frequency-division-multiple-access (FDMA) is used to divide the spectrum up into a plurality of radio channels corresponding to different carrier frequencies. In time division multiple access (TDMA) systems, carriers may be divided into time slots, as is done, for example, in the digital advanced mobile phone service (D-AMPS) and the global system for mobile communication (GSM) standard digital cellular systems. Alternatively, multiple users can use a common range of frequencies using spread-spectrum techniques as is typically done in code-division multiple-access (CDMA).
A typical digital communications system 19 is shown in FIG. 1. Digital symbols are provided to the transmitter 20, which maps the symbols into a representation appropriate for the transmission medium or channel (e.g. radio channel) and couples the signal to the transmission medium via antenna 22. The transmitted signal passes through the channel 24 and is received at the antenna 26. The received signal is passed to the receiver 28. The receiver 28 includes a radio processor 30, a baseband signal processor 32, and a post processing unit 34.
The radio processor 30 typically tunes to the desired band and desired carrier frequency, then amplifies, mixes, and filters the signal to a baseband. At some point the signal may be sampled and quantized, ultimately providing a sequence of baseband received samples. As the original radio signal generally has in-phase (I) and quadrature (Q) components, the baseband samples typically have I and Q components, giving rise to complex, baseband samples.
The baseband processor 32 may be used to detect the digital symbols that were transmitted. It may produce soft information as well, which gives information regarding the likelihood of the detected symbol values. The post processing unit 34 typically performs functions that depend on the particular communications application. For example, it may convert digital symbols into speech using a speech decoder.
A typical transmitter is shown in FIG. 2. Information bits, which may represent speech, images, video, text, or other content material, are provided to forward-error-correction (FEC) encoder 40, which encodes some or all of the information bits using, for example, a convolutional encoder. The FEC encoder 40 produces coded bits, which are provided to an interleaver 42, which reorders the bits to provide interleaved bits. These interleaved bits are provided to a modulator 44, which applies an appropriate modulation for transmission. The interleaver 42 may perform any of a number of types of interleaving.
The modulator 44 may apply any of a variety of modulations. Higher-order modulations are frequently utilized. One example is 8-PSK (eight phase shift keying), in which 3 bits are sent using one of 8 constellation points in the in-phase (I )/quadrature (Q) (or complex) plane. Another example is 16-QAM (sixteen quadrature amplitude modulation), in which 4 bits are sent at the same time. Higher-order modulation may be used with conventional, narrowband transmission as well as with spread-spectrum transmission. The Enhanced Data Rates for Global Evolution (EDGE) standard generally uses Gray mapping from triplets to 8-PSK symbols. As a further example, the Global System for Mobile communications (GSM) typically uses non-linear modulation which can be approximated by a binary linear modulator with a heavy partial response.
It is also known to provide a communication system using a plurality of selectable transmission schemes. For example, the EDGE standard provides for 2 modulation schemes, GMSK and 8-PSK, and a family of punctured convolutional codes of various rates. A normal data packet under this standard typically occupies 4 bursts from 4 consecutive frames. A choice of modulation and coding determines the payload of a packet. For example, low rate coding and GMSK typically produce a small payload and can operate at a low Signal to Noise Ratio (SNR). High rate coding and 8-PSK typically produce a large payload and can operate at a high SNR. The typical measure of quality used in the context of data is packet error rate, or frame error rate (FER). That is, a packet may be considered in error if one or more information bits from the payload is in error.
A further known approach to improving communication quality is frequency hopping, which may be viewed as a form of diversity transmission/reception. For example, the EDGE standard typically supports burst by burst frequency hopping, so that a data packet may effectively experience four independent fades. Thus, frequency hopping may provide for lower error rates in communications.